Abstract

This thesis develops the concept of an in situ electrolytic processor, a machine with electrochemical functions that purifies aqueous fluids at the point of production, whether from a bore water supply or effluent from a tannery. Where the contaminants in an effluent are useful they are separated in a specialised electrolyser for re-use in the process that produced the original effluent. The value in doing this exceeds the cost of electrolytic processing for tannery effluent. The functions of an electrolytic purifier were resolved into: flotation by bubbles, flocculation by corrosion of aluminium anodes, electrowinning by cathodic plating, disinfection, oxidation-reduction and pH modification. Improved understanding of the control of these functions has led to the ability to design better electrolysers because the functions were combined in a form that was appropriate to the required purification process. A link between extra-faradic corrosion of an aluminium anode and pH modification is postulated.

The most effective model, designed during the course of the project and described as the Flume, incorporated a novel corroding anode composed of thin pieces of aluminium, water flow in a cathodic flume, and a cheap water-porous membrane to separate the electrolyte into anolyte and catholyte. The novel anode was designed to improve clearance of corrosion products by maintaining a fast flow speed in proximity to the zone of anodic corrosion. The Flume model was used in an extensive test at a tannery. Greater than 90% of chromium in tannery effluent was removed by a combination of electroflocculation and electroflotation, at a lower cost than by treatment using standard chemical flocculent, using a side-stream Flume processor at a tannery. A smaller scale Flume model was used to test mechanisms of treatment in the laboratory using synthetic tannery effluent.

When treating alkaline effluent, the separating membrane in the Flume model enabled production of alkali at the cathode and production of acid at the anode. This was used for pH modification of effluent and, where the majority of the flow was anolyte, was able to produce a catholyte of pH greater than 13 from a wide range of inflow pH. The caustic catholyte is a valuable by-product of the electrolytic processing, especially when the upstream processes require net input of alkali (as is the case at a tannery or kraft pulp mill) and more generally because downstream biological treatment processes benefit from receiving neutralised effluent.

The degree of pH moderation of the whole outflow compared to the inflow was found to be controllable by adjustment of cell voltage. This effect enables treatment of effluent with variable pH.

Optimisation of an electrolyser for energy recovery, by both reduction of electrode over-potentials and electrolyte resistance, was incorporated in the full-scale designs. Based on a feasible cell voltage of 5 V, 20% recovery of the energy input is expected.